a relative abundance of large populations and therefore readily found in sedimentary rocks in the period of their existence;

evolved rapidly: thus have a very limited time duration and allow to achieve high accuracy in dating.

The best-known guide fossils are:

For the Palaeozoic (542 million to 251 million years ago): Trilobites, Graptolites, primitive Ammonites (Ammonoidea) and microfossils (Conodonts and Fusulinid).

For the Mesozoic (251 – 65.5 million years ago): Ammonites (Ceratitiidae and Ammonitida, such Arietites) and other microscopic fossils (eg. benthic and planktonic Foraminifera for dating within the Cretaceous, Tintinnids).

For the Cenozoic (65.5 million years ago – until now): microfossils, especially benthic and planktonic Foraminifera (Nummulites) and limestone nanofossils.